Pneumatic system responsive to changes in a variable condition



III

Jan. 19, 1960 P. F. K. ERBGUTH 2,921,595

PNEUMATIC SYSTEM RESPONSIVE T0 CHANGES IN A VARIABLE CQNDITION FiledFeb. 28, 1955 4 Sheets-Sheet 1 LOW PRE$$URE\ or M: rm

PAUL F. K. ERBGUTH IN VEN TOR.

Jan. 19, 1960 P. F. K. ERBGUTH 2,921,595

PNEUMATIC SYSTEM RESPONSIVE T0 CHANGES IN A VARIABLE CONDITION FiledFeb. 28, 1955 4 Sheets-Sheet 2 mllllllllfifp IIIIIIIIIA'! PAUL E K.ERBGUTH INVENTOR.

Jan. 19, 1960 P. F. K. ERBGUTH PNEUMATIC SYSTEM RESPONSIVE TO CHANGES INA VARIABLE CONDITION Filed Feb. 28, 1955 4 Sheets-Sheet 3 H/GH PRESSUR20/bs. PER .90. IN.

j 520 LOW PRESSURE PAUL EK. ERBGUTH INVENTOR.

A To EY Jan. 19, 1960 P. F. K. ERBGUTH 2,921,595

PNEUMATIC SYSTEM RESPONSIVE To CHANGES IN A VARIABLE CONDITION FiledFeb. 28, 1955 4 Sheets-Sheet 4 PAUL 5K. ERBGUTH INVENTOR.

A O E) United States Patent PNEUMATic SYSTEM RESPONSIVE T0 CHANGES IN AVARIABLE CONDITION Paul F. K. Erbguth, Great Neck, N.Y., assignor, by

mesne assignments, to Daystrom, Incorporated, Murray Hill, N.J., acorporation of New Jersey Application February 28, 1955, Serial No.490,907 11 Claims. (Cl. 137-85) ments for selectively positioning and/orcontrolling a remotely-positioned member, etc. Such condition-sensingmeans include a vane, or baflle, movable between a pair of alignednozzles across which flows an elastic fluid, generally air, Movement ofthe vane in the air stream inhibits, more or less, the flow of air fromthe discharge nozzle to the receiving nozzle resulting in correspondingvariations in the air pressure in an air relay connected as thereceiving nozzle. The resulting air pressure vari-' ations in the outputof the air relay efiects a desired operation such as a control of signalmeans, or a follow-up of air at low pressure between said nozzles, adevice responsive to changes in a variable condition and carrying a vanemovable into and out of the air stream, an air relay responsive to thevariations in air pressure in the receiving nozzle, and means responsiveto the output of the air relay to effect a movement of the vane until abalance is restored in the system.

An object of this invention is the provision of a pneumatic systemcomprising a discharge nozzle and an axiallyaligned receiving nozzle,means for maintaining a flow of air at'low pressure between the nozzles,a device responsive to changes in a variable condition and carrying avane movable into and out of the air stream, an air relay responsive tovariations in the air pressure in the receiving nozzle as the vane movesfurther into or out of the air stream, a bellows controlled by the airoutput of the air relay, and means movable by the bellows to effect amovement of the vane in a direction to maintain the system in balance;'

An object of this inventionis the provision of an electro-pneumaticsystem comprising axially-aligned disaction effecting return of the vaneto, or maintenance of the vane at, substantially its initial positionrelative to the aligned nozzles. Such operation is accompanied, ifdesired, by a simultaneous indicating, measuring or recording function.7

Prior systems of this type are of complex construction and, moreimportantly, are not adapted for actuation by a condition-sensing meansdeveloping only a low torque. 'An object of this invention is theprovision of a pneu-1 matic system responsive to changesin acondition-sensing device developing only a low torque and which systemprovides a measurement, record or control of a variable condition. a

An object of this invention is the provision of a pneumatic systemresponsive to changes'in a' variable condition andin which a vane ismovable between axially aligned air nozzles, said vane being movable bya low torque element such as the pointer of an electricalmillivoltmeter.

An object of this invention is the provision of a pneumatic system inwhich low pressure air, not exceeding 3" of water, passes from adischarge nozzle to an axially-. aligned receiving nozzle unlesscut offby a vane movable between the nozzles inlresponse to changes in avariable condition, the variation in the air pressure in the receivingnozzle effecting the operation of suitable mechanisms to provide ameasuring, recording or control function.

An object of this invention is the provision of a pneumatic system inwhich the position of a pair of axiallyaligned nozzles may be adjustedwith respect to an intercepting vane inorder to effect a desiredoperation of the yste An object of this invention isthe provision of apneumatic system comprising a discharge nozzle and an axiallyelignedreceiving nozzle, meansfor maintaining a flow charge and receivingnozzles, means maintaining a flow of air at low pressure between thenozzles, an electrical instrument responsive to changes in an electricalquantity, a pivotally-mounted arm mechanically-coupled to the pointerand carrying a vane movable in the air stream in response todisplacement of the pointer, an air relay responsive to changes in theair pressure in the receiving nozzle as the vane moves further into orout of the air stream, a bellows movable by the air output of the airrelay, a second pivotally-mounted arm mechanicallycoupled to thebellows, and resilient spring means coupling together the said.pivo'tally-mounted arms, the

arrangement being such thata movement of the vane effects an operationof the system to restore the vane substantially to its initialposition.v

These and other objects or the advantages of the invention will becomeapparent from the following specification when taken with theaccompanying drawings. It will be understood, however, that the drawingsare for purposes of illustration and are not to be construed as, defining the scope or limits of the invention, reference being had for thelatter purpose to the claims appended hereto.

In the drawings wherein like reference characters denote like parts inthe several views: i Figure l is a diagrammatic view of the electric andpneumatic circuits in an instrument embodying my invention; Figure. 2 isa diagrammatic view similar to Figure l but showing another embodimentof this invention;

Figure 3- is a diagrammatic view corresponding to the left hand endportion ofrFig urc 2 but showing a modifi cation; l p,

Figure'4 is an enlarged axial sectional view of the cooperating nozzlesof a preferred shape, viewed as on the line IV-'-IV of Figure l, in thedirection of the arrows; Figure 5 is a sectional view on the line VV ofFigure 4, in the direction of the arrows;

Figures 6 and 8 are axial sectional views of nozzles as in Figure 4 butshowing modifications;

Figures'7 and 9 are, respectively, a transverse sectional view on theline VIIVII of Figure 6, in the direction of the arrows and an endelevational view of the form of Figure 8;

Figure 10 illustrates a practical embodiment of my invention; V

Figure 11 is a perspective view of a portion of the mechanism of Figure10; and I Figure 12 is a diagrammatic view of the electricand bearings27. Each'of thejpivots 25 and 26 has secured thereto aspiral hairspring, :28 and 29, resiliently causing the coil 24 to occupy a givenposition with respect to thepoles of anassociatedpermanent magnetinotshown)- until electric current is passed therethrough. Such current maybe generated by ai -thermocouple 31, which from, as by means of a weight47 carried by a pan 48 may se'rveas a temperature-sensing element as,forexam ple, in the'furnace 40 shownin'Figure 12.

-; A pair of axially-aligned nozzles 32 and33; forming apneumaticcouple, are: positioned so that the leading edge of the vane 21 isnormally disposed therebetween. The vane 21 is desirably as illustrated,that is, formed of thin'metaland has an edge beveled, or formedbyadiagonal plane, to provide aknife edge to thereby facilitate themovement of the vaneinto an air stream, The

nozzle 32 is connected to a source of elastic fluid, under' pressure,such as a compressed air supply, through a tube 34. The tube '34 has aflow wrestrictor 35 therein to cause the air-to pass from the dischargenozzle 32 at a low pressure, say-about 1 of Water and no more than 3 ofwater, whichis apressure of about 1l.lb. .per.

suspended from one end of the lever 22a.

A pair of aligned nozzles 32a and 33a forming a pneumatic couple arepositioned so that the leading edge of the vane 21a is normally disposedin the air stream between the nozzles. Thisvane is desirably formed ofthin metal and has an edge beveled to a knife edge to thereby facilitatethe movement of the vane into the air stream. The nozzle 32a isconnected to a source of air under -.pressu.re through tube 34a,which'tube'has'a flow restrictor 35:2 therein to cause the to-flow from.the nozzle 32aat a low pressure, say about apressure of :1 and no morethan a pressureloffi" of water. The receiving nozzle 33a is connected tothe chamber of the relay 36a by 'tube37'a.

When the vane 21a is. moved, a variable amount of air flows into thereceiving nozzle 33a, thereby causing a; movement of the airrelaydiaphragm 38a. Assuming thatsuch vane movement results in anincrease inthe quantity of air flowing .to thereceiving nozzle 33a,there results an increase in air pressure Within the air relay chamber.The diaphragm 38a,-therefore, moves upward- 'ly thereby rotating the.connected member, suchas the pivoted lever 39a, in acounterclockwisedirection. The free endof the lever 39a carries.aflapper 51 cooperating with a nozzle. 52 that is connected to thesource of air pressure by the tube 53 containing a flow restrictor 54therein. 7 q 1 7 It may .be pointed out that the pressure ofithe .airsuppliedto thenozzle 52 maybe relatively high, that is, in the order ofpounds per squareinch and the actual airpressure in the tube 53dependsupon the spacing of the flapper '51 fromthe nozzle 52. Ontheother" hand,

the pressurelof the air suppliedto the discharge nozzle 32ais veryilow,the order of a fraction of a pound per square,inch,.thereby reducing toan insignificant degree the reaction eflects between the airstream'flowingacross the change in air. pressure within the chamber moves theair relay diaphragm 38, thereby moving a connected member, such as thepivoted lever'39 of a switch 41, so that itmay engage one or theother ofthe upper and lower stationary contacts .42 and 45. The engagement ofthe movable member with either of the contacts'42,- completes a circuitfor energizing one or the other of the lamps 43, '46 from the battery44.

Y 'From-the foregoing descriptiomlit will be seen that l have devised asystem whereby movement of the pointer ance point effects a desiredcontrol of 'signal lamps so that when the vane changes position betweenthe nozzles one lamp is de-energized and another energized. It will benoted that in this arrangement there isno follow-up action,[that is,there is merelya change in thesignal indi cation as the vane moves toeither side tion between the opposed nozzles.

" Those skilled in this-art will understand that; theprese't position ofthe vane'edge in the air stream may be altered of a preset posialignednozzles 32a, 33a and the .vane, 210:. This is im portant .sincemysystems are adapted for use with con- 40' dition-sensing devicesproducing very low torques, such as sensitive electricalinstruments,sensitive balances, etc,

:When the, air relay diaphragm moves the flapper '51 a greater distanceaway from the associated nozzle 52 there is effected a correspondingdecrease in the air pressure in the bellows assembly 55 (connected tothe output 'tube 53) causing a partial collapse thereof. 'In theparticular. arrangement illustrated in the drawing, the bellows 55ar'ebiased by a compressed spring 56 disposed between the upper surfaceof {the bellows and a fixed surface 57.

-A connecting wire 59 is wrapped around the drum. 58, one end of thewire'be'ing secured to the drum and the respect to thejshaft .2,6r z,jan'd normally held resilientlyin a'certainfp'osition as' byrmeans of ahair spring 63; The

inner endof the springf63is connected to the shaft 61 and as byadjusting the'normal zero position of th'e'in'strumen't pointer or "by aphysical setting of the aligned nozzles along" the arcuatepath of travelof the vane.21.'

Referring now to the embodiment of my invention illustrated in Figure 2,there is shown a modified pneubut in which a followup action is providedto return the vane to a throttling position between the aligned nozzles,such vane position being precisely that position. in which the entiresystem is in balance. .In this embodiment I show a pneumaticforce-balance system which may measure, record or control forces such asweight, torque or humidity. In this instance, the vane 21a 'is'carriedby a balance arm 22a that is secured to a pivoted shaft 26a, rotatablein the bearings 27a'and normally held'in one position by a hair spring74. "The lever 22a, therefore, 'tends to' stay in the positionillustrated until movedtherethe outer end "thereof is connected to" afixed abutment 64. The shaft6l has'a rigid angular member 65extending'towardfthe,'first vshaft 26a andtothe'end .of which the'outerportion of the hair spring 74is connected, as indicated at '66...Consequently, a partial collapseof the bellows 55 effects a rotation ofthe shaft 61 to'act on the hair spring '74 and tend to restore thevane'Zla to its throttling position between the aligned nozzles 32a and3301, thereby restoring the system to a new balance point.

It will now be apparent that mean pressure in the lzagllows 55 is ameature of theforce applied to the lever a.

I In addition to the restoration of the shaft 22ato its throttlingposition, the movement of the bellows may be measured by a carriedpointer 67, movable overacalibrated scale fitl to thereby-indicatethemagnitu de of the particular weight I 47. This factor may also .berecorded on a chart 69, "carriedlby 1a rotatingshaft 71, :as by apen 72carried by an arm 73 extending from-the shaft 61 and into engagementwith the chart. I

Referring now to the embodiment of the invention illustrated in Figure3, there is shown instead of the lever 22a, the pointer 22b of anelectrical measuring instrument such as a millivoltmeter. The vane 21ais attached to the pointer and is movable between the aligned nozzles32a, 33a to effect operation of the system in response to a change inthe electrical quantity controlling rotation of the instrument movablecoil 24 as, for example, the thermocouple 31. The remainder of theapparatus may be the same as illustrated in Figure 2.

As an alternative, however, the tube 37a may be connected directly to abellows, such as indicated by the reference character 55 in Figure 2,the pressure being indicated on a gauge 75. However, if this embodimentis adopted, it is necessary that the bellows, like 55, be relativelylarge because of the low operating pressure of the air within the tube37a. Forthis reason, it is generally better to operate the bellows froman air relay such as that designated 36a in Figure 2.

Referring now to Figures 4 and 5 there is shown one form of the nozzles32 and 33, and the-vane 21 which may be employed in any or all of thedescribed embodiments of my invention. To facilitate manufacturingthereof, these nozzles 32 and 33 are formed as tapered members havingapertures 76 smaller than the bore of the air-carrying tubes 34 and 37,so as to form a stepped construction. The cutting, or leading, edge ofthe vane 21 is desirably beveled, as indicated at 30. In other words ithas an approximately 60 degree knife edge, as indicated. 7

' The direction of movement of the vane 21, further into the air stream,from what may be the normal position illustrated in Figures 4 and 5wherein the vane cuts off part of the air stream, is in the direction ofthe arrow 50. The full-line position of the vane is normal to thedirection of air fiow, from right to left as viewed in Figure 4.However, to further facilitate the cutting of the air stream by the vane21, the plane of the vane may be turned slightly at an angle to theposition illustrated by the dotted lines in Figure 4.

Referring now to Figures, 6 and 7, there is shown a modified form ofnozzle 33c wherein the aperture at the tip, instead of being circular,is transversely elongated as indicated at 76 in Figure v7. Thiselongation in the tip portion 77 is formed by flattening the end of thetube. When nozzles of this form are used the vane may move in adirection either longitudinally of the elongated aperture, transverselythereof, or any intermediate relation. In the first instance, the cutoff action is much more gradual and less sensitive than in the secondinstance, as will be understood.

Figures 8 and 9 illustrate an embodiment in which the nozzle aperture76a, elongated or circular as desired, is formed in a cap or tip portion77a, which is in turn connected to the carrying tube 37 by solder orother suitable means.

Referring now to the embodiment of this invention illustrated in Figures10 and 11, there is shown a practical instrument embodying preferredfeatures shown diagrammatically in the preceding embodiments. Themechanism for sensing changes in a variable condition is here anelectrical measuring instrument, such as a millivoltmeter ormilliammeter, generally designated 23d. This instrument is more or lessconventional so the parts are not described fully. Sufiice it to saythat the part 78 is a soft-iron yoke or housing having an axial bore inwhich rotates a coil 24d mounted on pivots operating in suitablebearings such as the upper jewel screw 25d shown in the drawing. Each ofthe pivots has a conventional spiral hair spring (not shown) resilientlycausing the coil 24d to stay in a given position with respect to thepoles of the permanent magnet core M until electrical current is passedtherethrough. Such current may be generated as described in connectionwith the preceding embodiments. The pointer secured to the coil isdesignated by the reference character 22d, the end of said pointermoving over a scale 79 and between suitable limiting stops 81 and 82.

The pointer 22d has a tail 83 extending away from the scale 79 and whichengages a lever 84 secured to a shaft 85 journalled in the bearings 86and 87 forming part of a supporting frame 88 (see particularly Figure11). The upper end of this shaft 85 has a knurled setting wheel 89,secured thereto as by means of a screw 91. The setting wheel 89 carriesa disengaging rod 92 for the lever 84 and a stop rod 93 for limitingturning movement of itself, as by engaging the frame 88. The shaft 85corresponds with the shaft 26b of Figure 3, in that it is operated by ashaft 94, corresponding with the shaft 61 in Figure 2, so that iteffects turning of the shaft 84 by means of a shaft extension 95, thefree end of which is connected to the outer end of a hair spring 96carried by the shaft 94. In the present instance also, the lever 84carries a vane 21d.disposed between the aligned nozzles 32d and 33d,rather than having the vane carried directly by the pointer 22d of theinstrument as in the previously-described embodiments.

The shaft 94 carries an arm 97 having a wheel 98 movable by a lever 59d,the latter corresponding to the wire 59 of Figure 2, in that it effectsturning of the shaft 94. This lever is pivoted at 99 and operated by abellows device 55d corresponding with that designated 55 in Figure 2,but in this case by a link 60 connected between the lower end of thebellows and a point intermediate the ends of said lever 59d, instead ofby a wire wrapped around a drum. There is also an air relay 36d,corresponding with that designated 36a in Figure 2, in which thediaphragm 38d is moved (when the vane 21d moves in the air streambetween the aligned nozzles 32d and 33d) in response to a change in airpressure in the tube 37d, thereby operating the lever 39d pivoted at49d, so as to move the free end thereof further from the nozzle 52d andthereby correspondingly decrease the pressure in the output tube 53d.

The operation of the apparatus illustrated in Figures 10 and 11 issimilar to that of the apparatus illustrated in Figures 2 and 3.However, the apparatus illustrated in Figures 10 and 11 is more thanmerely diagrammatic. In this latter apparatus, air is received from themain supply tube 101 through a shut-off valve 102 where it passes to amanifold or box 103, such air being normally at a pressure of 20 lbs.per square inch, as is commonly available. From the manifold 103 thereextends a pipe 104 to the high-pressure gauge 105, from whence airpasses to a manifold or box 106.

From the manifold 103 there extends a tube 34d to the discharge nozzle32d, the pressure of the air in this tube being very low, as hasalreadyv been described. Thus, the operation of the apparatus, heredisclosed more in detail, corresponds with the operation of theapparatus of Figure 3, in that the vane 21d is movable between thealigned nozzles 32d and 33d by corresponding movement of the pointer22d, although not directly by said pointer.

In other words, the pointer tail 83 which extends in a directionopposite to the scale 79, engages an angular extension 107 on the lever84 and, on upscale movement of the pointer, moves it counterclockwise,as viewed in Figures l0 and 11. This effects a corresponding movement ofthe vane 21d further into the air stream between the aligned nozzles 32dand 33d. When the vane cuts further into the air stream, the pressure inline 37d is reduced, thereby causing a corresponding collapse of the airrelay diaphragm 38d. The lever 39d, therefore, moves toward the nozzle52d, thereby correspondingly increasing the pressure of the air in theoutput tube 53d and bellows system 55d, and causing the lever 5911 torotate counterclockwise. This rotation effects turning of the vane 21dfrom'the air stream to its initial throttling,

position, wherein the forces of meter torque and the. air system are. inbalance. The foregoing description presents the functional operation ofthe apparatus, but those skilled in this art will understand that. thevane is always.

partially in the air stream (see Figures 4' and 5)., that is, inthethrottling position when the actuating pointer is in the mechanical'zero position. Consequently, any slight movement of the pointer resultsin an immediate change 7 ingthe relay output and a correspondingrebalancing force is applied to said pointer.

To illustrate the control function of my invention, reference is nowmade to Figure 12, wherein there are shown the components required forthe control of tem-' perature in a furnace. 40. The relay and" chart.components correspondwith the form ofv my invention illus-' trated inFigure 2, and described inv connection there,- with, but thecondition-sensing device-is an electrical millivoltmeter as shownin-Figure Such'instrument deflects in accordance with the voltagefdeveloped'by the thermocouple 31 which in turn depends upon the:temperature of the furnace 40. v

The pressurecontroller 108, here .used, has an operating pressure rangegenerally equal to the air pressure range of the airrel ay 36a, that is,3-15 pounds pe'rlsquare. inch. The air relay tube 53is connected-to thecontroller by a tube 116'. Relatively highpressure airis-supplied to'the controller-through the tube 109 and thecontrolled air passes throughthe tube -1-11-to the control-valve"1=12 inserted in the fuel line 113of the furnace. The inputand' output air pressures of thecontroller-are-indicated-by the air supply pressure gauge 90and thecontrolled air pressure gauge 100. i

The, systemoperates in the following manner; With an increase ordecrease of furnace temperature, the electricaloutput of thethermocouple. in the furnace changes. This change is sensed by themoving coil 2i-(Fig ure"3) and motion is transmitted to the vane 21a bypointer zib. The movement of this vane, as previously described,properly alters the position of the air relayflapper51 relation to theassociated nozzle 52, thereby resulting in a corresponding movement ofthe bellows 55' and. the recorder pen 72. V I ,In addition to theseindicatingand recording funetions,

I the air pressure change ,in the tube 53 is transmitted throughthe tube110 to thelsensing mechanism of-the controller 108. The controllersenses this airfpressure change and corrects the openingoftheavalve 112toreturn the furnace temperature to thedesired value. H Those skilled inthis art will understand-thatthe desired-;oper ating temperature ofsthefurnace may be setby rotating: the knob 144 of the controller-sensingmechanism to -.the proper value as indicated byan associated-scale-105calibrated "in terms of temperature. It'is here pointed out that mysystem does not depend uponthe particulartype of pressure,.controllerused. The particular controller used will depend upon theparticular application and may be one of numerousconventional types suchas asimple on-ofi, high sensitivity, full throttling, reset, or=etc.

changes and modificationsasha i 1 W t h S p d spirit of theinventionaifiSl'Iqqilcdl in the following claims.

'Iclaimz. v a 1. A pneumaticsystem comprising in combination, a sensingmember movabledn "response to Ch g in variable condition, a. discharge nz an ia ly receiving nozzle. spaced from the discharge nozzle, meansnormally maintaining a. low pressur H air stream between the saidnozzles, a vane movable in the. air stream in response to movement ofthe sensing member and thereby to vary the amount of airpassing into-thereceiving nozzle, a tube connected to a high pressure air supply, an airrelay connected to the receiving noajzle and responsive to variations inthe air pressure Within the receiving nozzle to produce correspondingvariations, in air pressure within the said tube, a bellows movable inresponse to variations in the air pressure within said tube, andmechanical coupling means connected between the bellows and the sensingmember and arranged to move the vane in a direction opposite to thatbrought about by a change in the sensing member. i

2. The invention as recited in claim 1, wherein the sensing member is abalance arm carried by a rotatable securedto the bellows.

shaft and said mechanical coupling means includes a second rotatableshaftco-axial andv in t ndem with respect.

to the first shaft, means coupling the two shafts together, spring meansnormallybiasing the. shafts'to a predetermined position, andmeanscoupling the second shaft to the said bellows to effecta turning ofthe shaft in response to'collapse or expansion ofthe bellows. a v

3. The invention asrecited .in claim *2, wherein the means coupling thesecond shaft to the bellows comprises a drum carried by the second shaftand a filamentary member entwined about the drum, said filamentarymember having one end securedto thedrum and the other end 4. Theinvention as recited in claim 1, wherein the sensing member is anelectrical instrument having a pointer to which the vane is attached.

5. The invention as recited in claim 1, wherein the sensing member is anelectrical instrument having. a pointer, and the vane is carried by arotatablc arm having an end lying in "the path of travel of the pointer.

6. An electro-pneumatic system' comprising an elec: trical instrumenthaving a pointer, a rotatable arm ha ing an end disposed in the path oftravel of the pointer, a vane carried by said rotatablearm, a dischargenozzle disposed on one side of the vane and'connected to a. low:pressureair supply, an axially-aligned receiving'nozzle' disposed on the otherside of the vane and receiving air from the discharge nozzle ex cept ascutoff-by the vane, vanIair relay operated by the variation inairipressurewithin' the receivingnozzle, an output tube connected to asource of air pressure, means operated by said air relay ,to varythe airpressure in said'output -t ube, a bellows movable in response tovariations in the air pressure in said'output tube, and means couplingthe bellows to the said rotatable arm carrying the'vane.

7. The invention as recited in claim 6, including. means adjustable tospace the said rotatable arm 'a predetermined distance fromthe pointerwhen the latteris'in its normal zero position.

8. The invention as recited in claim 6, wherein the means coupling thebellows 'to the rotatable arm comprises a shaft rotatable about therotational axis ofthe said rotatable arm, a lever pivoted at one end andcon nected to thebellows at a pointintermediateof. its ends, a rigid armsecured to the said'shaft and engaging the said lever,.a resilientspring having one end secured to the said shaft, and a rigid rnembersecured to theother ,end ofthespring and to the saidrotatable arm. 7

9. An electro-pneumatic system comprising an electrical instrumentincluding a pointer cooperatingwith a calibrated scale, a rotatable armhaving an end lying in the path of travel of the pointer tail,axially-aligned discharge and receiving nozzles, a tube connecting thedischarge nozzle to a constant pressure air supply having a pressure notexceeding three inches of water, a vane carried by the rotatable arm andmovable between the nozzles to vary the quantity of air flowing from thedischarge nozzle to the receiving nozzle, an air relay having a chamberclosed by a diaphragm, a first tube connecting the receiving nozzle tothe said chamber whereby the diaphragm will move in response to changesin the quantity of air passing into the receiving nozzle, a second tubeconnected to a source of air pressure exceeding one (1) pound per squareinch, means operated by the said diaphragm to vary the air pressure inthe second tube in accordance with the variation of air pressure in thesaid chamber, a bellows movable in accordance with the air pressurevariations in the said second tube and means coupling the said bellowsto the rotatable arm carrying the vane.

10. The invention as recited in claim 9, wherein the means coupling thebellows to the rotatable arm comprises an extension carried by the saidrotatable arm, a lever pivoted at one end engaging the said extensionand a link connected between an end of the bellows and a pointintermediate the ends of said lever.

11. A pneumatic system comprising in combination, a discharge nozzleconnected to a source of low air pressure, an axially alined receivingnozzle spaced from the discharge nozzle so that air from the lattertends to flow thereinto, a vane element with an edge portion beveled toprovide an approximately knife edge at a portion normally lying in theair stream between said nozzles, and the plane thereof turned slightlyso that the flow of air thereagainst has a component aiding said vaneelement in cutting into the air stream.

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